PURPOSE: To enable the min. suppression of the change in the output of a detec tor due to offset flow by making the magnetic flux distribution in the detector non-uniform inclusive of a cross-section containing an electrode and a pipe axial direction, by providing a means for reducing a weighing function to the electrode output of an electrode peripheral part.

CONSTITUTION: When the change in a flow speed is generated in the vicinity of electrodes 3, 4, the change in detected electromotive force becomes large as compared with other area and, in order to obtain electromotive force accurately proportional to a flow speed even with respect to offset flow, it is necessary to reduce the value of the weighing function. Therefore, notches 11 are provided to upper and lower magnetic electrodes 7, 8 for supplying magnetic flux to the vicinities of the electrodes 3, 4 and, by these notches 11, magnetic flux density is rapidly reduced outside the right angle plane of a pipe axis including the mount surface of the electrodes 3, 4 but increased in the axial direction before and behind said right angle plane and, by this effect, the values of the weighing function are lowered both in the pipe cross-section and pipe axial direction of a conduit 1.